Various types of operating circuits are known to start and operate compact fluorescent lamps. One type of circuit is illustrated in the FIG. 7 schematic of French Publication No. 0346782. This schematic is similar, in general principle, to the state of the art as practised in a lamp sold by the Osram Company under the registered trademark "DULUX EL" or in the lamp of the Philips Company which bears the denomination "PLC 20 Electronic". Using the circuit of French Publication 0346782 as an example of such circuits, after the two input terminals of the DC/AC converter (or oscillator) are energized by a DC voltage which appears across a filter capacitor, the starting capacitor C5 charges through a starting resistor R3 to a voltage which is substantially equal to the threshold voltage of the threshold element (i.e., the diac). The threshold element breaks down and supplies a pulse to the base terminal of transistor T2. As a result, transistor T2 begins to conduct. A current flows through transistor T2 and the load circuit. Subsequently, this transistor becomes non-conducting and the other transistor T1 becomes conducting. This process is then continuously repeated. This leads to an oscillation, i.e., an alternating current through the load circuit which includes the discharge tube.
U.S. patent application Ser. No. 07/664,161 describes some disadvantages which may appear in some circuits similar to those described above. For example, when power to the circuit is removed, a momentary blink or flicker in the lamp may occur immediately after the tube is extinguished. It was observed that when AC power to the circuit is removed, a voltage initially remains on the filter capacitor of the DC power supply. This filter capacitor voltage gradually depletes to a point (usually greater than the starter threshold voltage) where the oscillator shuts down. However, the starting capacitor is allowed to recharge to a point where the threshold element of the starting circuit triggers causing the oscillator to conduct for a short period of time. Consequently, the discharge tube will blink or flicker as a result of current from the filter capacitor flowing through the conducting transistors and load circuit. This conduction continues for approximately 10 msecs. until the filter capacitor voltage is less than the starter's trigger voltage.
Another problem, which may appear in circuits similar to those described above, is at the end-of-lamp life when the emissive material on one or both of the filament electrodes has depleted. Although a discharge is unable to be established between the lamp electrodes, the oscillator may continue to conduct current through circuit components causing an unnecessary consumption of power until, for example, the AC power source is disconnected or the lamp and tank capacitor are removed from the load circuit. In the instances where the lamp is permanently connected to the circuitry, such as in an integral lamp unit, this latter option is unavailable.
U.S. patent application Ser. No. 07/664,161 proposes the use of fusible-type base drive resistors. As a result of the increased current flow caused by the non-operable lamp, one of the fusible resistors creates an open circuit and thereby permanently inhibits operation of the oscillator.
While the use of fusible resistors to permanently inhibit the oscillator upon end-of-lamp life is indeed effective, it may be undesirable to implement this approach with a lamp unit containing a replaceable lamp without having made other provisions. It is apparent that following end-of-lamp life, the oscillator in a replaceable lamp unit must remain functional in order to accommodate a replacement lamp.
U.S. Pat. No. 4,554,487, which issued to Nilssen on Nov. 19, 1985, describes a subassembly which disables the inverter in case the inverter output power remains at an excessive magnitude for more than a very brief period. The input of the subassembly includes a voltage-clamping device (e.g., a varistor) coupled in parallel with the tank capacitor of a series-resonant LC circuit so as to limit the voltage developed thereacross. Also included in the subassembly is a current sensing circuit for sensing current flow through the varistor. The output of the subassembly is connected directly to the junction of transistor Q2 and the secondary winding CT2 of positive feedback current transformer CT2. While such disabling circuitry may effectively prevent the inverter from self-destructing in case the fluorescent lamp fails to start or if the lamp is removed from the circuit, several disadvantages still exist with this approach. For example, the subassembly of Nilssen requires a high voltage clamping device (e.g., a varistor) which may be relatively expensive. Moreover, once the inverter illustrated in FIG. 1 of Nilssen has stopped oscillating, the inverter will not restart until power line voltage is removed and then reapplied at a later time (i.e., after much of the charge on the filter capacitors has had a chance to leak off). It may be advantageous to be able to replace a failed lamp without having to remove the power line voltage.
Another approach is described in U.S. Pat. No. 4,644,228, which issued to Nilssen on Feb. 17, 1987, wherein a control means provides a short circuit across the tank capacitor in the event the lamp fails to conduct within about 25 milliseconds. After about 1.5 second, the short circuit is removed for about 25 milli-seconds, thereby permitting the voltage across the tank capacitor to grow to a magnitude sufficient to ignite and operate the lamp. If lamp current does not then flow, or if at any time it ceases to flow, the short will be reapplied within 25 milli-seconds. Thereafter, until power is removed or until an operable lamp is connected, the control means will continuously repeat the cycle of 1.5 second short circuit and 25 milliseconds open circuit. While such control means, which periodically provide a short circuit across the tank capacitor, may operate effectively without requiring the removal and reapplication of power in order to replace a failed lamp, the control circuit requires the use of high voltage components, such as varistor V, bridge rectifier BR and transistor Qa in order to withstand the high lamp starting voltage.